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Self-heating (SH)-induced electrical and optical switching in high quality VO2 films grown by magnetron sputtering on a c-cut sapphire substrate has been investigated under various applied constant current pulses (ID). The effect of SH on the behavior of electrical conductivity (σ), optical transmittance ( $$\tilde{T })$$ , and film temperature (T) examined by applying a constant current pulse of various magnitudes with a pulse duration of five seconds (Δt = 5 s) in VO2 films which were pre-heated to and stabilized at 57 °C, at the brink of insulator-to-metal transition (IMT). The SH effect that arose from the application of constant ID pulses led to a significant increase in T and substantial changes in σ and $$\tilde{T }$$ . Observations showed that, depending on the magnitude of ID, the σ and $$\tilde{T }$$ demonstrate strikingly different temporal behavior not only during the SH-induced IMT but also after the removal of ID. The observed phenomena could be explained using a simple model based on percolation theory previously proposed for the present system. The outcome of the model is confirmed by the results of the structural IMT obtained by Raman micromapping.

This study focuses on the effect of the substrate temperature (

The general fabrication process for stabilized amorphous selenium (a-Se) detectors is vacuum deposition. The evaporant alloy is typically selenium alloyed with 0.3–0.5%As to stabilize it against crystallization. During the evaporation, fractionation leads to the formation of a deposited film that is rich in As near the surface and rich in Se near the substrate. The As content is invariably not uniform across the film thickness. This paper examines the effect of non-uniform As content on the charge collection efficiency (CE). The model for the actual CE calculation is based on the generalized CE equation under small signals; it involves the integration of the reciprocal range-field product (the schubweg) and the photogeneration profile. The data for the model input were extracted from the literature on the dependence of charge carrier drift mobilities and lifetimes on the As content in a-Se1−xAsx alloys to generate the spatial variation of hole and electron ranges across the photoconductor film. This range variation is then used to calculate the actual CE in the integral equation as a function of the applied field. The carrier ranges corresponding to the average composition in the film are also used in the standard CE equation under uniform ranges to examine whether one can simply use the average As content to calculate the CE. The standard equation is also used with ranges from the spatial average and average inverse. Errors are then compared and quantified from the use of various averages. The particular choice for averaging depends on the polarity of the radiation-receiving electrode and the spatial variation of the carrier ranges.

Stabilized amorphous selenium (a-Se)-based digital flat-panel X-ray imaging detectors are widely used in modern mammography. The dependence of the hole lifetime, $\tau _{h}$ , in a-Se on high-dose X-ray irradiation under high electric fields is examined. The X-ray-induced effects on the samples were studied using conventional time-of-flight (TOF) and interrupted-field time-of-flight (IFTOF) techniques. The samples were placed under a high electric field (5–10 V/ $\mu \text{m}$ ) and exposed to X-ray irradiation, after which TOF and IFTOF measurements were taken to find the hole drift mobility and lifetime after X-ray exposure. There was no observed change in the hole drift mobility but there was a large drop in the hole lifetime. The reduction in the hole lifetime depended only on the total or accumulated dose and the applied field. There was no dependence on the dose rate, over a range of 0.12–2.5 Gy/s, or on the X-ray photon energy, over a range of 50–90 keVp (corresponding to a mean photon energy of 31.9–44.7 keV). The implications of these findings can be seen through the calculation of the charge collection efficiency (CCE). The results show that the effects of X-ray irradiation on a-Se X-ray detectors are considerable when the detector is under an applied field during exposure, but are minimized when the operating field is high (corresponding to a high CCE) and the a-Se is of high-quality electronic grade material.

Stabilised amorphous selenium (a-Se) is currently used in the majority of direct conversion mammographic X-ray imaging detectors due to its X-ray photoconductivity and its ability to be uniformly deposited over large area TFT substrates by conventional vacuum deposition. We report experimental results on photocurrent spectroscopy (frequency-resolved spectroscopy (FRS) and single-time transients), on vacuum-deposited a-Se films. We show that all measured photocurrents depend critically on the relative time spent by the material in the light and in the dark. We identify that the observed pronounced variation in optical response depends on the density of trapped (optically injected) charge within 200 nm of the surface and show that it is the ratio of dark and light exposure time that controls the density of such charge. Our data confirm that the localised charge radically influences the photocurrent transient shape due to the effective screening of the applied field within 200 nm of the surface. The field modification occurs over the optical extinction depth and changes both the photogeneration process and the drift of carriers. Many aspects of our data carry the signature of known properties of valence alternation pair (VAP) defects, which control many properties of a-Se. Modelling in the time domain shows that light generation of VAPs followed by optically triggered VAP defect conversion can lead to near-surface charge imbalance, demonstrating that VAP defects can account for the unusual optical response. The stabilised a-Se films were deposited above the glass transition temperature of the alloy with composition a-Se:0.3% As doped with ppm Cl. Electron paramagnetic resonance measurements at temperatures down to 5 K did not detect any spin active defects, even under photoexcitation above band gap.

Radiophotoluminescence (RPL) is a phenomenon in which a luminescence centre is formed in a medium upon interaction with ionizing radiation. The RPL is observed by a conventional photoluminescence (PL) technique, and the PL intensity is proportional to the dose. With the latter feature, the RPL has found successful applications in personnel and environmental dosimetry. However, the conventional materials considered for radiation measurements may be limited to Ag-doped phosphate glass, LiF, and Al2O3:C,Mg. The recent research works, however, have found a number of additional RPL materials for luminescence dosimetry. The aim of the present paper is to review the series of newly reported RPL materials and potential applications in dosimetry.

Electrical and optical properties Cs-doped a-Se0.95As0.05 (stabilized a-Se that has been alloyed with As) have been investigated. As expected there was no electron paramagnetic resonance signal on Cs-doped films or bulk samples, which put the spin-active defect concentrations below 1015 cm−3. The Cs-addition to a-Se0.95As0.05, leads to the n-type doping of a-Se0.95As0.05 in the sense that the undoped material has μhτh>> μeτe whereas the alkaline doped material has μeτe>> μhτh. The Cs addition also leads to a reduction of the refractive index n and a reduction of the glass transition temperature Tg, and affects the temporal relaxation behavior of a-Se film thickness after annealing and sequential quenching. We have measured the refractive index dispersion, n(λ) versus λ, bandgap (Eg) and Urbach width (ΔE) for undoped and Cs-doped films at room temperature and at a temperature just below the glass transition temperature. The photoluminescence (PL) experiments confirm earlier experiments that the PL emission is a broad emission spectrum with a significant Stoke’s shift following roughly the ~ Eg/2 empirical rule. The present work confirms that Cs-doped and As-stabilized a-Se is n-type.

Self-assembled single crystal VO2 microrods (MRs) with a length up to 600 µm have been synthesized on Si and sapphire substrates by the annealing of V2O5 thin films. The nucleation and growth of VO2 MRs from V2O5 thin films were investigated. The morphology and microstructure evolutions of the intermediate phases during the reduction process were characterized by SEM, HRTEM, XPEEM, and Raman spectroscopy. The results have shown that the conversion of V2O5 thin films to VO2 MRs is dominated by a melting-nucleation-growth mechanism. The growth of VO2 MRs can be controlled by V2O5 melting process and a consistent feeding of V2O5 liquids to the growing VO2 MR would lead to ultra-long VO2 MRs. Further, epitaxial growth of VO2 MRs can be induced by using an r-cut sapphire substrate.

NbOx thin films have been deposited on silicon (100) and quartz substrates by magnetron sputtering using a metallic Nb target in an optimized argon–oxygen atmosphere. This work investigates the dependence of structure–property relations on the key deposition parameters towards establishing optimum deposition conditions for the growth of NbOx polycrystalline films. It is found that a sputtering condition corresponding to DC power density 9.87 W cm−2, a substrate temperature of 720 °C, low gas pressures of 8 mtorr, a target to substrate distance of 45 mm gives thin films with good homogeneity and a high degree of crystallinity in the case of both NbO2 and Nb2O5. X-ray diffraction (XRD) and Raman spectroscopy confirmed the tetragonal phase of NbO2 and orthorhombic phase of Nb2O5 for similar deposition temperatures. Scanning electron microscopy (SEM) observations indicate that NbO2 has a unique nanoslice structure while Nb2O5 has a flake-like structure. The optical transmittance of the films has been investigated and found to be dependent on the oxygen gas content during deposition; the optical transmittance decreases with increasing O2 gas content. Optical constants of the films were calculated by fitting a suitable thin film transmittance model to experimental transmittance spectra using a modified Swanepoel technique. The nanohardness and stress in the films were measured by nanoindentation and an optical profilometer respectively. Nanohardness and stress in the film show no large dependence on the oxygen gas content except at high oxygen gas content. The nanohardness value of NbO2 films is approximately 6 GPa, and the Young’s modulus is 150 GPa. The Nb2O5 films exhibit a nanohardness of 5.8–13 GPa and a Young’s modulus of 137–161 GPa.

Ion implantation of Sm-ions has been tested in fabricating 2D detectors for microbeam radiation therapy (MRT). Sm-ions have been successfully implanted into fluoroaluminate (FA) glasses. The implantation concentration was chosen to be 5 × 1015 ions/cm2 and the ions were implanted at an energy of 2 MeV. After implantation, samarium ions resided within a thin plane very near the surface in the glass, which is expected to be beneficial for 2D imaging. Following implantation, photoluminescence (PL) spectra indicate that the embedded Sm-ions are in the form of Sm2+ and Sm3+. Subsequent annealing around the glass transition temperature (475 °C) converts all Sm2+ ions into Sm3+. Under X-ray irradiation, a partial conversion of Sm3+ into Sm2+ has been observed which may be used as measure of the X-ray dose delivered into the sample. QFRS (quadrature-frequency-resolved-spectroscopy) measurements on PL prominent emissions from Sm3+ and Sm2+ ions show that the PL decays associated with various transitions are in the 0.1 to 100 ms range (slow transitions). X-ray irradiation has led also to the appearance of broad and intense photoluminescence bands associated with X-ray induced structural defects in the host glass as confirmed in the unimplanted FA glasses. The generation of hole trapping centers in the host glass leads to the capture of photogenerated holes and thus allows the electrons to convert Sm3+ to Sm2+. Defect related PL decay signals were measured to be in the nanosecond region. These unwanted defect related fast decaying signals have been separated from slow Sm2+ and Sm3+ photoluminescence signals by using an “out-of-phase” PL measurements through a phase-sensitive photodetection technique with a modulated excitation laser diode and a lock-in amplifier. Overall, the Sm-ion implanted fluoroaluminate glass shows the successful conversion from the trivalent form of samarium (Sm3+) to the divalent form (Sm3+) under X-ray irradiation over a large dynamic range of X-ray intensities (800 Gy in air).

Active matrix flat panel imagers (AMFPIs) with thin film transistor (TFT) arrays are becoming the standard for digital x-ray imaging due to their high image quality and real time readout capabilities. However, in low dose applications their performance is degraded by electronic noise. A promising solution to this limitation is the Scintillator High-Gain Avalanche Rushing Photoconductor AMFPI (SHARP-AMFPI), an indirect detector that utilizes avalanche amorphous selenium (a-Se) to amplify optical signal from the scintillator prior to readout. We previously demonstrated the feasibility of a large area SHARP-AMFPI, however there are several areas of desired improvement. In this work, we present a newly fabricated SHARP-AMFPI prototype detector with the following developments: metal oxide hole blocking layer (HBL) with improved electron transport, transparent bias electrode for increased optical coupling, and detector assembly allowing for a back-irradiation (BI) geometry to improve detective quantum efficiency of scintillators. Our measurements showed that the new prototype has improved temporal performance, with lag and ghosting below 1%. We also show an improvement in optical coupling from 25% to 90% for cesium iodide (CsI) scintillator emissions. The remaining challenge of the SHARP-AMFPI is to reduce the dark current to prevent dielectric breakdown under high bias and further increase optical quantum efficiency (OQE) to CsI scintillators. We are proposing to use a newly developed quantum dot (QD) oxide layer, which shows to reduce the dark current by an order of magnitude, and tellurium doping, which could increase OQE to 85% to CsI at avalanche fields, in future prototype detectors.

The research on nitridophosphate materials has gained significant attention in recent years due to the abundance of elements like Mg, Zn, P, and N. We present a detailed study of band gap and electronic structure of M

Concrete has been widely used in pavements and buildings, and it is necessary to increase its hardness in order to resist deformation, penetration, and abrasion for these applications. This study explores the effects of addition of CuO and/or TiO2 nanoparticles on the hardness of cement mortar. Scanning electron microscopy, X-ray diffraction, energy dispersive spectroscopy, and Rockwell hardness testing were used to study the microstructure, chemical and phase composition, and hardness. The results showed that the addition of nanoparticles can effectively improve the hardness of cement mortar by improving the microstructure and hydration process. This is because they offer additional nucleation sites for hydrates to deposit on, producing a more compact microstructure with finer grains. As the concentration of nanoparticles added increases, the enhancement is more obvious and stable. However, after the concentration reaches its maximum level, the hardness starts to decrease due to the formation of defects, mainly voids, caused by excess nanoparticles. CuO and TiO2 nanoparticles show similar effects on the microstructure and hardness, but the addition of both CuO and TiO2 nanoparticles with similar amounts shows denser microstructure and higher hardness as they densify the composites.

In this work we report on a study of a series of zinc-borophosphate (ZBP) glasses doped with neodymium oxide (Nd2O3). The synthesis of the samples was performed by a high temperature melt quenching procedure in an air atmosphere. We present a complex photoluminescence study on a series of (72.31-x) ZnO-18B2O3-9.69P2O5-xNd2O3 glasses doped with trivalent neodymium oxide (with concentration x) in a range 0.15–1.00 mol%) to explore possible applications. The excitation and luminescence properties were measured, and the photoluminescence was studied within the spectral range from 200 nm to 950 nm under different excitation wavelengths. Emission spectra in the 850–1500 nm range were obtained under an excitation wavelength of 808 nm. We have also measured the thermal properties by differential scanning calorimetry (DSC). A strong emission peak is observed at 1060 nm with a width of 30 nm and an exponential decay with a characteristic time of 95 μs. The results of a series of Nd-doped zinc-borophosphate glasses show potential as a medium for solid state infrared laser applications.

Radiophotoluminescence (RPL) properties of Sm-doped CaSO4 for radiation dosimetry applications are reported. The samples with varying Sm concentrations are prepared via the solid-state reaction process. The as-prepared samples show photoluminescence due to typical 4f–f transitions of Sm3+ whereas, after X-ray irradiation, additional emission features appear with a broad band peaking at 630 nm as well as a set of multiple sharp lines across 680–820 nm, which are attributed to the 5d–4f and 4f–4f transitions of Sm2+, respectively. Therefore, the RPL in the present material system relies on the generation of Sm2+ centers. The sensitivity is about 3 times lower than that of Ag-doped phosphate glass, but no fading and build-up of signal are evident even immediately after the irradiation. The signal is reversible by heat-treatment at 500 °C, and is reproducible even after the thermal erasure, especially when the differential signal between pre- and post-irradiation is taken into account.

We have prepared Sm:MgF2 ceramics by the spark plasma sintering technique and studied the radio-photoluminescence (RPL) properties. RPL is a phenomenon seen in selected phosphors in which a new photoluminescence (PL) emission centre is generated by ionizing radiations such as X-rays. In the present system, RPL is observed with two different emissions in the blue and near infrared (NIR) ranges due to M(C2h) and M(C1) centres. The dynamic range using the former centre is in the high dose range above 100 Gy while the former centres have a sensitivity below 100 Gy. In addition, heat-treating an irradiated 0.01%Sm:MgF2 sample induces the reduction of the Sm3+-ion, i.e. Sm3+ → Sm2+). These centres are metastable, and their concentration can be increased or decreased by heat-treatment and exposing to UV light.

Vanadium dioxide (VO2) thin films of different thickness have been deposited on Si substrates by using DC magnetron sputtering. The effects of substrate pre-treatment by means of seeding (spin coating and ultrasonic bathing) and biasing on the structure and optical properties were investigated. Seeding results in a smaller grain size in the oxide film, whereas biasing results in square-textured crystals. VO2 thin films of 150 nm thick show a near-zero IR transmission in switched state. Especially, the 150 nm thick VO2 thin film with seeding treatment shows an enhanced switching efficiency.

In this research we have found that Sm-doped BaF 2 -Al 2 O 3 -B 2 O 3 glass ceramics show radio-photoluminescence (RPL) properties associated with X-ray irradiation. Before X-ray irradiation, the photoluminescence (PL) emission is only due to the 4f-4f transitions of Sm 3+ observed around 600 nm; however, after X-ray irradiation it shows additional PL emissions due to the 4f-4f transitions of Sm 2+ . We attributed the origin of this RPL due to the inter-valence conversion of Sm ion (Sm 3+ → Sm 2+ ). The glass ceramic sample includes BaAlBO 3 F 2 crystallites in the glass matrix, and the RPL is only valid for Sm included in the crystalline phase since the precursor glass (without the crystalline phase) does not show RPL. The RPL response is so stable that it does not show any indication of fading even by heating at high temperatures up to 400 °C. For radiation sensing applications, we have confirmed that it shows a monotonically increasing response with X-ray dose at least over the 10–10,000 mGy; and it has been demonstrated for 2D dosimetry applications.

Optical properties of vacuum deposited films of (a) pure a-Se (b) a-Se:0.5%As, (c) a-Se:6%As, (d) a-Se:6%As doped with Cs at the ppm level (140–220 ppm) have been measured from the transmission spectra over the range from the visible to the IR, covering 450–2500 nm. There is an increase in the refractive index of pure a-Se upon alloying with 6%As and a further increase with ppm-level Cs doping. There is a slight reduction in the bandgap with 6%As alloying. All four samples were subjected to x-irradiation at 30 and 70 kVp (21.56 and 37.6 keV average photon energies) with corresponding incident doses of 15.4 and 44.8 Gy in air. As a result of X-ray irradiation, there is an increase in the refractive index and a decrease in the film thickness (increase in the density), which confirms that the changes in n stem primarily from changes in the film density as expected from the Clasius-Mossotti equation. Upon the cessation of X-ray irradiation, n and d were observed to change, that is recover, towards their pre-exposure equilibrium values over a time scale of hours. After about 13 h of resting in the dark, n and d were found to have recovered to their equilibrium values within experimental errors. We also examined the time evolution of n, d, E U (Urbach bandgap) and δE (Urbach width) when a-Se:6%As doped with ppm-level Cs is brought to 75 °C, annealed there for 10 min and then brought back down to room temperature (23.5 °C). It was found that n and d follow opposite trends as expected from the Clausius–Mossotti equation, that is, the changes in n stem primarily from changes in the density. There is an increase in d and a drop in n when the sample is annealed at 75 °C. As soon as the sample is brought back to room temperature n and d slowly recover towards their equilibrium values and after a few hours, n and d values are indistinguishable from the original equilibrium values within experimental errors. Thus, n and d (or density) exhibit typical glass relaxation behavior. At 75 °C, δE is larger and E U is smaller. Upon cooling, both E U and δE show relaxation towards their equilibrium values similarly to n and d. The Wemple—DiDomenico model was also used to model the refractive index dispersion data and the parameters E o and E d were determined for all the samples before and after 70 kVp irradiation. It was observed that there is a slight increase in E d upon X-ray irradiation for all the samples. The examination of the thermal stability of a-Se and a-Se:0.5%As films shows that even after very high doses, several thousands of Grays deposited into a-Se, the Hruby thermal stability coefficient is unaffected and there is no permanent deterioration in the stability.

We report on the optical properties of Er-doped As3S7 chalcogenide films prepared using the two step dissolution process utilizing the As3S7 glass dissolved with propylamine and by further addition of the tris(8-hydroxyquinolinato)erbium(III) (ErQ) complex acting as an Er3+ precursor. Thin films were deposited by spin-coating, thermally stabilized by annealing at 125 °C and further post-annealed at 200 or 300 °C. The post-annealing of films at 200 °C and 300 °C densifies the films, improves their optical homogeneity, and moreover activates the Er3+:4I13/2 → 4I15/2 (λ ≈ 1.5 μm) PL emission at pumping wavelengths of 808 and 980 nm. The highest PL emission intensity was achieved for As3S7 films post-annealed at 300 °C and doped with ≈1 at% of Er which is beyond the normal Er3+ solubility limit of As–S melt-quenched glasses. The solution-processed deposition of the rare-earth-doped chalcogenide films utilizing the organolanthanide precursors has much potential for application in printed flexible optoelectronics and photonics.

In this work, single crystal VO2 (A) microbelts (MBs) were synthesized for the first time via the reduction of V2O5 thin films without a catalyst and without a hydrothermal process. The morphology and microstructure of the as-prepared VO2 (A) MRs was characterized by SEM, fluorescence mapping, Raman, XRD, XPS and FTIR. Results show that the as-prepared VO2 (A) MBs exhibit belt-like morphology with a rectangular cross section. The formation mechanism was proposed as melting of V2O5 thin film, VO2 nucleation in V2O5 droplets, VO2 growth. Light scattering measurements were carried out at varying sample temperatures. The results found that the as-prepared VO2 (A) MBs exhibit optical switching properties at a high temperature around 170 ℃, which is promising for designing thermally stable optical switching devices.

Vanadium oxides (VO x ) have been widely studied as “smart materials” because of their capability of going through a reversible metal-insulator-transition. They are of considerable technological interests for applications in optoelectronics, ultrafast optical switches, electrochomic devices, and lithium microbatteries. However, vanadium-oxygen system is complicated due to the multivalency of vanadium, which makes preparation of VO x with single stoichiometry difficult. Therefore, structural characterization of vanadium oxides of different stoichiometriesis highly desirable and would provide helpful guideline to both materials preparation and their structural characterization. In the present work, VO x thin films with different stoichiometries under various bonding states were successfully prepared by reactive sputtering with and without post oxidation or reduction and characterized using Raman spectroscopy. Characteristic Raman spectra of single and multi- valence states of VO x including V 2 O 3 , VO 2 , V 6 O 13 , and V 2 O 5 are presented and discussed. The results have demonstrated that high purity VO x thin films with single stoichiometry can be obtained under well controlled conditions.

Rare-earth doped phosphors have been used in many applications including radiation measurements. In the latter applications, the radiation photons are converted to light so that we can indirectly detect the radiation using a conventional photodetector. In this work, we have prepared and characterized a Ce-doped SiO 2 glass for dosimeter applications. Unlike conventional techniques such as sol-gel synthesis, the sample was prepared by spark plasma sintering. Although the PL emission seems to be only from the Ce 3+ ions in the structure, due to the X-ray induced luminescence, we have also observed optically-stimulated luminescence (OSL), and thermally-stimulated luminescence (TSL), owing to a pair of silylenes and a set of dioxasilirane and silylene in addition to Ce 3+ . We have measured the detector response vs irradiation dose for both the OSL and TSL. The detector response in both cases is linear over the dose range from at least 1 mGy to 2 Gy. Particularly, the sensitivity of TSL is so high that it should be considered to be a good candidate for practical applications.

Radio-photoluminescence (RPL) is a phenomenon seen in luminescent materials in which the appearance of new photoluminescence (PL) emission is induced by an incident ionizing radiation such as X-rays; and the signal is stable even after the irradiation and during the PL measurement. Since the induced PL intensity is proportional to the irradiation dose, the RPL can be used in radiation measurements. The distinct advantage of RPL over the conventional thermally- or photo-stimulated luminescence (abbreviated as TSL or PSL) dosimeters is the stability of response signal. With an RPL detector, it allows us to readout the signal multiple time without signal fading. In this work, we discovered that CsBr:Sm showed an RPL phenomenon by X-ray irradiation, and we characterized this new material as an RPL detector. While the sample showed PL emissions mainly in the visible range, after an X-ray irradiation additional emissions could be observed in the red to near-infrared range around 650–850 nm and 900–1000 nm and longer. The RPL response was fairly stable overall, but very interestingly the 650–850 nm signal slightly increased while the 900–1000 nm decreased during PL readout. The dynamic range was confirmed over 1–104 mGy with linear response.

In the present paper, we have investigated hole transport in stabilized a-Se films using interrupted-field-time-of-flight (IFTOF) experiments with interruption times up to 600 μs. A distinct advantage of IFTOF measurements is that one can monitor the average “free” hole concentration p(t) (= p(x,t) averaged over the thickness of the sample L) at a given location x1 in the sample inasmuch as the applied field is removed at a certain time t1 for an interruption period of ti. At time t1 + ti, the field is reapplied and the recovered photocurrent i2 at t = t1 + ti is measured with respect to the original photocurrent i1 at t = t1. The experimental results are interpreted by the comparison of experimental photocurrent transients with numerical and Monte-Carlo simulations of multiple trapping hole transport for different DOS models; and their agreement with a featureless DOS distribution in the vicinity of valence band is confirmed. The examination of IFTOF experiments (i2/i1 as a function of ti) for very long interruption times puts the energy position of deep traps controlling the hole lifetime to be above 0.65 eV from Ev. The results are critically discussed in view of past experiments on the DOS in a-Se and recent structural modeling work on defects in the structure. Further, the work in this article vindicates the use of shallow trap controlled mobility and a single deep trapping time in the modeling of hole transport in recently developed a-Se based X-ray detectors; that is we only need shallow and deep traps to model the detector performance.

Vanadium dioxide (VO2) thin films were deposited on silicon (100), quartz, and r-cut sapphire substrates by DC magnetron sputtering at 650 °C. The thin films were characterized by Raman spectroscopy and imaging, scanning electron microscopy (SEM), X-ray Photoelectron Spectroscopy (XPS), X-ray Diffraction (XRD), Energy Dispersive X-ray Spectroscopy (EDS), and Transmission Electron Microscopy (TEM). The optical transmittance and hence the insulator-to-metal transition (IMT) and metal-to-insulator transition (MIT) were characterized as a function of temperature. The effects of substrate material on VO2 thin film’s growth, microstructure, stoichiometry and optical properties were investigated. The results show that the as-deposited VO2 thin films on Si, quartz, and sapphire all show a near-zero IR transmission in switched metallic state; especially VO2 thin film grown on sapphire shows superior IMT characteristics compared with previously published works on thin VO2 films, and very close to its bulk single crystal form, which is probably due to the significantly reduced imperfections in the film and the interface, and especially the highly preferred growth with similar paralleled grain orientation of VO2 on sapphire arising from both the high deposition temperature and epitaxial growth. The spectral transmittance of the films on quartz and sapphire substrates was analyzed to extract the optical constants n and k as a function of wavelength. No significant difference between optical properties could be observed for films deposited on quartz and sapphire substrates indicating that the differences in microstructure of these films play a minor role in their optical properties. n and k spectra are, in general, in agreement with previously reported works. The absorption coefficient versus photon energy characteristics indicate that VO2 in the insulating phase possesses a direct bandgap at 2.6 eV and an indirect bandgap of 0.52 eV. Raman imaging was used to map the phase mixture of various regions of the film at micron scale (1.2 μm × 1.2 μm) during heating and cooling scans through the IMT and MIT. The dependence of the integrated Raman intensities at 220 cm−1 and 600 cm−1 on the temperature shows an excellent correlation with the optical transmittance through the IMT and MIT, both during heating and cooling scans. It is shown that the transformation occurs by the growth of one phase at the expense of the other and, in the IMT/MIT region, both phases coexist.

Electrical conductivity (σ) and optical transmittance of high quality VO2 thin films deposited by DC reactive magnetron sputtering on r-cut sapphire substrates (at 650 °C) have been measured simultaneously as a function of temperature by heating and cooling scans through the phase transition region. The partial concentration of the metallic phase (Xm) has been calculated from the optical transmittance, and the σ(Xm) dependence has been analyzed through an insulator-to-metal transition (IMT) during heating and through a metal-to-insulator transition (MIT) during cooling. The results have shown to be consistent with the Efros–Shklovskii percolation theory, predicting the formation of two-dimensional infinite conductive cluster (ICC) during IMT and the preservation of three-dimensional ICC during MIT. The critical concentrations (Xc) corresponding to the appearance of ICC at IMT and the disappearance of ICC at MIT were found to be very different, 0.57 and 0.06, respectively. A mathematical model explaining very small Xc at MIT was developed. The dissimilarity of the ICC topology during IMT and MIT is connected with the appearance and disappearance of local mechanical stresses imminent in VO2 phase transitions.

The research on nitridophosphate materials has gained significant attention in recent years due to the abundance of elements like Mg, Zn, P, and N. We present a detailed study of band gap and electronic structure of M2PN3 (M = Mg, Zn), using synchrotron-based soft x-ray spectroscopy measurements as well as density functional theory (DFT) calculations. The experimental N K-edge x-ray emission spectroscopy (XES) and x-ray absorption spectroscopy (XAS) spectra are used to estimate the band gaps, which are compared with our calculations along with the values available in literature. The band gap, which is essential for electronic device applications, is experimentally determined for the first time to be 5.3 ± 0.2 eV and 4.2 ± 0.2 eV for Mg2PN3 and Zn2PN3, respectively. The experimental band gaps agree well with our calculated band gaps of 5.4 eV for Mg2PN3 and 3.9 eV for Zn2PN3, using the modified Becke-Johnson (mBJ) exchange potential. The states that contribute to the band gap are investigated with the calculated density of states especially with respect to two non-equivalent N sites in the structure. The calculations and the measurements predict that both materials have an indirect band gap. The wide band gap of M2PN3 (M = Mg, Zn) could make it promising for the application in photovoltaic cells, high power RF applications, as well as power electronic devices.

VO2 microrods (MRs) have been successfully synthesized by the reduction of V2O5 thin films. The effects of reduction temperature and thickness of the precursor V2O5 thin films on the stochiometry and morphology of the resulting VO2 MRs have been studied in this work. Results show that size of the VO2 MRs increases with increasing precursor thickness due to the enlargement of the droplets during the melting of V2O5 thin films. Further, the growth of VO2 MRs from V2O5 droplets on a sapphire substrate form a unique angle between each other, which is probably due to epitaxial growth. Reduction temperatures between 600 °C and 650 °C are preferred for the growth of VO2 MRs, while temperatures lower or higher than this range produce V2O5 nanostructures or VO2 nanoparticles, respectively. HRTEM shows that during VO2 MR’s growth, the (110) plane remains parallel to the Si substrate, and a 10 nm amorphous interface was formed between the VO2 MR and the Si substrate. Optical properties of the MRs were examined by carrying out light scattering experiments as a function of temperature to identify the temperature range of insulator-to-metal (IM) and metal-to-insulator (MI) transition characteristics. The optical scattering experiments show the large influence of microrod (MR) size and distribution on the characteristics of MI and IM transitions. The transition behavior can range from polycrystalline film type transition to new unique features.

Microbeam radiation therapy (MRT) utilizes highly collimated synchrotron generated x-rays to create narrow planes of high dose radiation for the treatment of tumors. Individual microbeams have a typical width of 30-50 µm and are separated by a distance of 200-500 µm. The dose delivered at the center of the beam is lethal to cells in the microbeam path, on the order of hundreds of Grays (Gy). The tissue between each microbeam is spared and helps aid in the repair of adjacent damaged tissue. Radiation interactions within the peak of the microbeam, such as the photoelectric effect and incoherent (atomic Compton) scattering, cause some dose to be delivered to the valley areas adjacent to the microbeams. As the incident x-ray energy is modified, radiation interactions within a material change and affect the probability of interactions, as well as the directionality and energy of ionizing particles (electrons) that deposit energy in the valley regions surrounding the microbeam peaks. It is crucial that the valley dose between microbeams be minimal to maintain the effectiveness of MRT. Using a monochromatic x-ray source with x-ray energies ranging from 30 to 150 keV, a detailed investigation into the effect of incident x-ray energy on the dose profiles of microbeams was performed using samarium doped fluoroaluminate (FA) glass as the medium. All dosimetric measurements were carried out using a purpose-built fluorescence confocal microscope dosimetric technique that used Sm-doped FA glass plates as the irradiated medium. Dose profiles are measured over a very a wide range of x-ray energies at micrometer resolution and dose distribution in the microbeam are mapped. The measured microbeam profiles at different energies are compared with the MCNP6 radiation transport code, a general transport code which can calculate the energy deposition of electrons as they pass through a given material. The experimentally measured distributions can be used to validate the results for electron energy deposition in fluoroaluminate glass. Code validation is necessary for using transport codes in future treatment planning for MRT and other radiation therapies. It is shown that simulated and measured micro beam-profiles are in good agreement, and micrometer level changes can be observed using this high-resolution dosimetry technique. Full width at 10% of the maximum peak (FW@10%) was used to quantify the microbeam width. Experimental measurements on FA glasses and simulations on the dependence of the FW@10% at various energies are in good agreement. Simulations on energy deposited in water indicate that FW@10% reaches a local minimum around energies 140 keV. In addition, variable slit width experiments were carried out at an incident x-ray energy of 100 keV in order to determine the effect of the narrowing slit width on the delivered peak dose. The microbeam width affects the peak dose, which decreases with the width of the microbeam. Experiments suggest that a typical microbeam width for MRT is likely to be between 20-50 µm based on this work.

In this paper, we report radio-photoluminescence (RPL) properties of Sm-doped LiCaAlF6 as a new RPL material. The material was successfully synthesized via the mechanochemical route by mixing LiF, CaF2, AlF3, and SmF3 raw powders in a planetary ball-mill. The resulting powder sample shows photoluminescence (PL) predominantly due to the 4f-4f transitions of Sm3+, indicating that the doped Sm ion is optically activated. After irradiating with X-rays, additional PL emissions appeared around 700 nm; that is, Sm-doped LiCaAlF6 exhibits RPL properties. From the spectral features and luminescence dynamics, the origin of the RPL centre is determined to be the 4f-4f transitions of Sm2+. Therefore, in this material system, intra-valence change of Sm ion (Sm3+ → Sm2+) is induced by X-ray irradiation. Furthermore, using an elementary detector and the RPL signal, we could detect signals under a dose as low as 100 mGy, and the RPL signal is reasonably stable (~2%) under UV excitation over 5 min. The latter stability is attributed to the thermal activation of trapped electrons. In addition, X-ray imaging is demonstrated by utilizing the RPL of LiCaAlF6:Sm as an imaging plate.

Stabilized amorphous selenium (a-Se) photoconductive layers are currently used in the majority of modern digital x-ray flat panel imaging detectors in mammography. We examine the effects of pre-exposure of a-Se to high-dose x-ray irradiation on both hole and electron lifetimes, τe and τh, respectively, without any field applied to the device. The x-ray irradiation was from an Al-filtered tungsten target x-ray tube. We show that reduction in τh and τe depends only on the total or accumulated dose, D, absorbed in a-Se, and not on the rate of dose delivery, dD/dt, over the range of 0.15 Gy/s–2.5 Gy/s or on the x-ray energy over 50–90 kVp, corresponding to a mean photon energy over 31.9 keV–44.7 keV. The x-ray induced effects on charge transport are independent of the x-ray intensity and x-ray photon energy but dependent on the total energy absorbed in a-Se. The latter finding allows x-ray induced drop in the carrier lifetimes to be simply and conveniently modeled by τo/τ = 1 + AD, where τo is the lifetime before x-ray exposure (equilibrium lifetime), τ is the lifetime after exposure, D is the absorbed total dose, and A is a constant, which is 0.203 (±0.021) Gy−1 for the hole lifetime and 0.0620 (±0.0090) Gy−1 for the electron lifetime, a factor of three smaller than that for holes. X-ray irradiation had no effect on hole and electron drift mobilities. Reduction in carrier lifetimes with the total dose was examined at 10 °C, 23.5 °C, and 35.5 °C, close to the glass transition temperature, where x-ray induced effects are stronger. A is independent of hole and electron lifetimes but has a strong temperature dependence, increasing sharply with temperature. After the cessation of x-ray irradiation, carrier lifetimes relax (increase) to their pre-exposed equilibrium values over time scales that depend on temperature. Recovery has been interpreted and analyzed in terms of considering the kinetics of the rate at which x-ray induced capture centers are removed, as the structure restores the equilibrium concentration of deep traps. The annealing process of excess hole traps has a fast and a slow decay component, with time constants τr1 and τr2, respectively. The recovery processes associated with τr1 and τr2 exhibit activation energies larger than those typically involved in electronic transitions and are comparable to bond breaking energies in Se–Se and Se–As. Near 35.5 °C, close to the glass transition temperature, τr2-recovery has a large activation energy, pointing to structural relaxation phenomena. Fast annealing (∼6 min) of excess hole traps at 35.5 °C is, in a general sense, in agreement with the disappearance of irreversible photoinduced effects and suppression of crystallization (strain relief) at the a-Se/substrate interface, as observed previously. In the case of recovery of the electron lifetime, single exponential decay in excess electron traps and clear activation energy of 1.91 eV/atom point to a probable Se–Se bond breaking involved in returning excess electron traps to equilibrium concentration. Interpretations based on x-ray induced excess valence alternation pair (VAP) and intimate VAP type defects are also considered, including conversion from neutral defects to charged VAP defects. The implications of the present findings on x-ray sensitivity of a-Se detectors through the charge collection efficiency (CCE) are also examined and discussed. An effective carrier lifetime concept is used to describe the effect of x-ray irradiation on carrier lifetimes, which is then used to find CCE in a pre-exposed a-Se detector. The results indicate that x-ray induced effects are negligible for nearly all practical applications of a-Se mammographic detectors in use provided that the detector is operated at a sufficiently high field and a-Se has sufficiently long initial lifetimes, i.e., it is a high quality electronic grade material.

We re-examine electron and hole transport in pure and stabilized amorphous selenium (a-Se) and attempt to construct a DOS distribution in the mobility gap below E c and above E v based on time-of-flight (TOF) transient photoconductivity measurements. First, we review the current status of a-Se, its recent use in commercial X-ray detectors, and the scientific information that is available in terms of its density of states (DOS). We review and describe a convenient multiple trapping transport mechanism to calculate the TOF transient photocurrents in a system with a distributed DOS in the mobility gap and invoke a number of assumptions that allow the photocurrent to be calculated as an inverse Laplace transform. The assumptions behind the model are critically examined. Then, by comparing the calculated TOF photocurrent shapes directly with experimental waveforms, the DOS distribution has been extracted in a-Se in the vicinity of conduction and valence bands (below E c and above E v ). Below the conduction band, the DOS distribution seems to demonstrate three peaks at around E c−0.30 eV, E c−0.45 eV and below E c−0.55 eV. First principles atomistic modeling provided insight into the origins of these peaks and the peaks have been assigned to certain valence–alteration pairs (VAPs) appearing in different configurations. In contrast to the conduction band vicinity, the DOS distribution above the valence band seems to be almost totally featureless. Within the range of experimental uncertainties, a featureless DOS below E v down to 0.55 eV seems to be able to explain a vast range of data, that is, it can explain hole transport not only as a function of the electric field and temperature down to 123 K, but also the thickness dependence of the mobility over two orders of magnitude. At room temperature, the hole transport is nondispersive but at temperatures below ~200 K, it becomes dispersive exactly along the lines argued by Pfister and Scher (Adv Phys 27:747, 1978), and the mobility evinces a clear thickness dependence. The first principles calculations yield two defect levels related to VAP type defects in this region of the mobility gap but the valence band tail is so broad that it masks these peaks. This seeming asymmetry between the conduction and valence bands may be related to their physical origins: the conduction band arises from antibonding states and valence band is due to interacting lone pairs.

The 4I9/2 -> 4I13/2 emission band of Er3+ is potentially interesting for CH4 detection because it can overlap with the optical absorption band of CH4. GaLaS and GeGaS glasses doped with Er3+ ions show the desirable emission under 808 nm excitation. An attempt of spectrally shifting 4I9/2 -> 4I13/2 emission band by substituting S by O or Se to obtain a perfect match to the absorption band of CH4 leads to the weakening and disappearance of the 4I9/2 -> 4I13/2 band. This phenomenon is tentatively related to the acceleration of non-radiative relaxation in the case of oxygen addition and to the suppression of up-conversion in the selenium substitution case. The relative intensity of the 4I9/2 -> 4I13/2 band (with respect to the 4I13/2 -> 4I15/2 band) varies significantly in powdered and bulk materials. This effect is discussed in terms of the radiation diffusion (i.e. radiation trapping) model and is confirmed by Monte-Carlo simulations.

Modern flat-panel x-ray imaging detectors have played an important role in the transition from analog to digital x-ray imaging. They capture an x-ray image electronically and hence enable a clinical transition to digital radiography. This chapter critically discusses the material, transport and imaging detector properties (e. g., dark current) of several potential x-ray photoconductors and compares them with an ideal photoconductor for use in direct-conversion imaging detectors. The present chapter also considers various metrics of detector performances including sensitivity, detective quantum efficiency, resolution in terms of the modulation transfer function, image lag and ghosting; and examines how these metrics depend on the photoconductor material, and detector structure and design.